This invention relates generally to oscillators having a surface acoustic wave device as a frequency stabilizing element and more particularly to surface acoustic wave oscillators which operate at high frequencies.
As is known in the art, surface acoustic wave devices utilize the propagation of ultrasonic acoustic waves in piezoelectric materials. In particular, such devices generally include a substrate having a surface which supports surface wave propagation and a pair of transducers disposed on said surface to couple to said surface wave propagation. Generally, each transducer is comprised of a pair of interdigitated conductive members having a first set of conductive stripes connected to a first terminal and a second set of said conductive stripes connected to a second, different terminal. When a voltage is impressed across the pair of terminals of a first one of said transducers, the voltage is propagated as a stress wave through the surface wave propagation surface, until it is coupled to the second transducer causing an output voltage across the pair of terminals of the second transducer. For typical piezoelectric materials, propagation velocities 10.sup.3 to 10.sup.4 m/sec are common. In general, due to propagation velocities of the acoustic waves and photolithographic limitations on the spacing of the interdigitated conductive stripes of the transducers practical surface acoustic wave devices are generally limited to operation in the range of 30-800 MHz.
Nevertheless, there is a need to operate SAW oscillators at even higher frequencies, such as up to X band (8 GHz to 12 GHz) and beyond for applications such as radar, electronic countermeasures and communication systems.
Present technique for extending the frequency of operation of SAW stabilized oscillators include the use of frequency multipliers at the output of the oscillator, use of electron-beam lithography to decrease spacing limitations and thus extend the frequency limits of SAW resonators, and the use of SAW devices which operate in a harmonic or non-fundamental mode of operation.
The foregoing approaches each present limitations in the usefulness of the technique to extend SAW oscillator frequency of operation. The use of a frequency multiplier at the output of the oscillator increases the system complexity of the oscillator by increasing the number of circuits required and may also increase the phase noise of the oscillator. Furthermore, due to the increase in the number of circuits, the reliability of the oscillator is reduced and the cost of the oscillator is increased. A problem with the use of e-beam lithographic techniques is the need for expensive specialized e-beam equipment which increases the cost of the SAW component in the oscillator. Further, throughput rates of e-beam lithography are relatively low compared to photolithography, thus adding to the cost of the SAW component. The problem with using the SAW resonator which operates in a non-fundamental (i.e. harmonic mode of operation) is that performance characteristics of such harmonic-mode devices are poor particularly for oscillator applications. Generally, harmonic-mode devices have relatively high insertion loss and relatively low Q compared with, for example, a fundamental-mode SAW resonator.
Finally, each of these approaches offers an increase in oscillator frequency by only a factor of 2 or 3 at considerable expense in the terms of cost and complexity. Accordingly, it would be desireable to provide a SAW stabilized oscillator which operates at higher frequencies by a factor of at least 2 or 3 and preferably more, and further which is less expensive than current approaches.